1. Field of the Invention
The present invention relates to a color cathode ray tube. In particular, the present invention relates to the shape of a frame for supporting a shadow mask, which is an element of a color cathode ray tube.
2. Description of Related Art
FIG. 6 is a perspective view showing a color selection mechanism of a conventional color cathode ray tube. As shown in FIG. 6, the color selection mechanism of the conventional color cathode ray tube includes a shadow mask 101, a pair of first frames 102 for stretching and supporting the shadow mask 101, and a pair of second frames 103 for supporting the pair of first frames 102 (see JP 2001-176420 A, for example).
As shown in FIG. 7, the first frame 102 is formed to have a substantially triangular cross-section and includes a bottom portion 105, an inclined portion 106, a joint portion 107 and a lateral portion 104 for supporting the shadow mask 101. The lateral portion 104 has an edge 104a for stretching and supporting the shadow mask 101, and this edge 104a has an arc shape when viewed along a normal direction of the lateral portion 104. Further, the entire surface of the joint portion 107 on the side of the lateral portion 104, which is to overlap the lateral portion 104, contacts the lateral portion 104. At an edge 107a of the joint portion 107 on the side of the shadow mask, the joint portion 107 is connected to the lateral portion 104 by welding.
This first frame 102 is formed as follows. After a metal plate is pressed and folded so as to have a substantially triangular cross-section, any adherent foreign substances are cleaned, and then the joint portion 107 and the lateral portion 104 are welded together.
Thereafter, the first frames 102 are welded to the second frames 103 as one piece. Subsequently, after a blackening treatment, the shadow mask 101 is stretched along the edges 104a of the lateral portions 104 of the first frames 102.
However, in the conventional color cathode ray tube, since the entire surface of the joint portion 107 of the first frame on the side of the lateral portion 104 is in contact with the lateral portion 104, a cleaning agent cannot enter the space between the lateral portion 104 and the joint portion 107 in the subsequent process of cleaning the first frame. Accordingly, foreign substances such as unwanted oil and dust are not cleaned but remain in the first frame, causing a problem of deteriorating high-voltage withstand capabilities of the color cathode ray tube. Moreover, the above-described remaining foreign substances have brought about a problem that poor welding occurs at the time of welding the lateral portion 104 and the joint portion 107.